The single stitch construction shows a single row of stitching securing the hem edges and sleeve cuffs of the garment. On most modern t-shirts the standard double row of stitching is now used.
Of course, many double-stitch t-shirts are both vintage and valuable. And many single stitch t-shirts are contemporary and common—many brands, from Ralph Lauren to Aeropostale, use an exaggerated single stitch construction to replicate a vintage look.
Brands and designers nowadays use the single stitch construction to replicate the Vintage aesthetic. Their thick lines of stitching secure slightly firmer and rigid fabric highlighting the difference in the original paper thin tee’s.